If a unit enter a hex with units locked into Melee from a previous turn, reinforcing it, and does so before the melee hex is activated in the current turn (to resolve the melee), the melee hex is immeditely "activated" and said melee is resolved. If the unit reinforce hte Melee hex after it was activated previously in the current turn, then the melee is not resolved again; it will have to wait until next turn, as a melee can only be fought (resolved) once per turn.